1. Field of the invention
This invention relates to a dielectric ceramic material. More particularly, it relates to a dielectric ceramic material which exhibits excellent dielectric characteristics, i.e., a high relative dielectric constant (hereinafter represented by .di-elect cons..sub.r) in a high frequency region, a high unloaded quality coefficient (hereinafter represented by Q.sub.u), and a small absolute value of the temperature coefficient of resonance frequency (hereinafter resonance frequency is represented by f.sub.0, and the temperature coefficient thereof is represented by .tau..sub.f). The dielectric ceramic material of the present invention is suited for use in multilayer circuit boards, resonators and filters particularly for use in a high frequency region, an impedance matching element for various microwave circuits, and the like.
2. Description of the Related Art
With the recent increase of communication information, rapid progress is being made in various communication systems utilizing the microwave region, such as mobile telecommunication systems, satellite communication systems, positioning systems using communication data, and satellite broadcasting. Use of the communication systems in a submillimeter wave region has been demanded. Many dielectric materials have been developed with the rapid progress. These dielectric materials are required to have (1) a high relative dielectric constant .di-elect cons..sub.r, (2) a high unloaded quality coefficient Q.sub.u (i.e., a small dielectric loss 1/Q.sub.u), and (3) a small absolute value of .tau..sub.f (i.e., small temperature dependence of f.sub.0).
In particular, a dielectric material used in a submillimeter wave region is required to have an especially high Q.sub.u, and it is desirable that .tau..sub.f be controllable freely around 0 ppm/.degree. C.
Microwave dielectric porcelain compositions based on Li.sub.2 O--CaO--Sm.sub.2 O.sub.3 --TiO.sub.2 are disclosed in JP-A-5-211007 and JP-A-5-211009. These materials have a particularly excellent .di-elect cons..sub.r value but a relatively small Q.sub.u. While the .tau..sub.f is relatively controlled by the composition, the precise control has been difficult, and control to nearly 0 ppm/.degree. C. has not been realized as yet.